Mass spectrometer



March 117, 1953 F, o msoN 2,632,110

MASS SPECTROMETER Filed July 20, 1950 2 SHEETS-SHEET 1 F/@ j A xMAGNET/C FIELD T0 ELECTRON BEAM COLLECTOR ELECTRODES l2 CVL/NDR/CAL IONBEAM Fig 3 ELECTRON GUN 42 'MZENET P0LE2I 28 M 2a I GAS lNLET 40COLLECTOR ELEC7RODE\ 26' 24 4 E 4- 38 AMPLIFIER & F i i i A 35 I kEcogER I To I 5; vAcuuM 32 32 PUMP t 5 25 i 33 5' /2.9,4 29 i i 50 I 20 E vMAe/vEr 0L2 2 TAR6E7'46/ ELECTRON BEAM INVENTOR.

F CHARLES F. ROBINSON BY hfl-T 0 C/LLATOR ATTORNEY March 17, 1953 c 'FROBlNSON MASS SPECTROMETER 2 SHEETSSHEET 2 Filed July 20, 1950 M m5 0 0m n C L INVENTOR. CHARLES E ROBINSON ATTORNEY Patented Mar. 17, 1953MASS SPECTROMETER Charles F. Robinson, Pasadena, Calif., assignor toConsolidated Engineering Corporation, Pasadena, Calif., a corporation ofCalifornia Application July 20, 1950, Serial No. 174,888

8 Claims. 1

This invention is directed to improvements in methods and apparatus forthe analysis of mix tures by mass separation. More particularly theinvention is concerned with that phase of mass spectrometry wherein massseparation is accomplished as a function of differences in theperiodicity of motion of ions of differing mass-tocharge ratio in amagnetic field.

The principle of mass spectrometry is, in general, one of spatiallyseparating ions produced from a sample to be analyzed as a function ofthe mass-to-charge ratio of the ions, and selectively collecting theseparated ions. Spatial separation of ions of differing mass-to-chargeratio may be accomplished in many ways, usually involving application ofmagnetic or electrical fields to induce and take advantage ofcharacteristic differences in movement of ions in such fields whichdifferences are a function of their massto-charge ratio. A collectorelectrode may be disposed in space so that under any given set ofconditions only ions of a given mass-to-charge ratio will impinge on anddischarge at the collector electrode.

It has been found that ions subjected to a magnetic field and analternating electrical field normal to the magnetic field will move inspiral orbits about an axis parallel to the magnetic field. It is alsoknown that in such crossed fields, ions of differing mass-to-chargeratio will exhibit different and characteristic periods of movementabout the axis as a function of the magnetic field strength. Ions of agiven mass-to-charge ratio having a periodicity of motion correspondingto the frequency of the alternating field will travel about the axis inorbits of ever increasing radius. These ions are referred to as resonantions. All ions of mass-to-charge ratio different from the mass-to-chargeratio of the resonant ions will travel about the axis in orbits, theradii of which increase to a maximum, collapse back to the axis and soon in successive cycles. These ions are referred to as non-resonantions. The nonresonant ions of differing mass-to-charge ratio will travelin different orbits and will attain different maximum radii with thoseions most closely approaching the mass-to-charge ratio of the resonantions attaining the greatest radial displacement from the axis.

By locating a collector electrode at a distance from the axis ofrotation exceeding the maximum orbital radius of the non-resonant ions,the resonant ions can be selectively collected and measured. To avoidanomalous ion paths within the field, ions are preferably introducedinto the field or are formed at some point along the aforementionedaxis. This is conveniently accomplished by projecting an electron beamthrough the crossed fields parallel to the magnetic field. The electronbeam then defines the axis of rotation of the ions.

The designation of ions of any particular massto-charge ratio asresonant or non-resonant is relative only to the operation variables,since any change in the magnetic field strength or in the frequency ofthe alternating field will result in ions of a different mass-to-chargeratio becoming resonant with the alternating field. For this reason, allor any portion of the total mass spectrum of a sample to be analyzed maybe scanned by varying the frequency of the alternating field or thestrength of the magnetic field so as to bring ions of diiferingmass-to-charge ratio into resonance With the alternating field.

In the practice of this form of mass spectrometry wherein ions areformed along an axial electron beam traversing the crossed magnetic andalternating fields, the number of ions formed varies directly with thecross sectional area of the electron beam. It follows that thesensitivity of the analysis is directly proportional to the crosssectional area of the electron beam. However, increased sensitivity isnot readily attained by simply increasing the radius of a more or lessuniform beam since to do so would tend to cause ions originating in thecenter of the beam to have excessive path lengths in reaching acollector means located at the periphery of the fields. As a result ofsuch excessive path lengths, ions originating in the center of an ionbeam of relatively large radius Will have a disproportion ately largechance of being lost by gas collision before they are collected. Suchloss will, of course, spoil the analytical accuracy of the method.

I have now found that the area of the electron beam may be increased toattain the desired high sensitivity without at the same time increasingthe probability of ionic collision of any of the ions originating at anyplace within the beam. To accomplish this, I propose to ionize the gasample within the region of the crossed magnetic and alternating fieldsby means of an electron beam in the form of a hollow cylinder. Such ahollow beam may be used without increasing the over-all ionic pathlengths and at the expense only of an increased magnetic gap area with aresultant many fold increase in sensitivity.

The invention therefore contemplates in mass spectrometry involving thespatial separation or ions of a given mass-to-charge ratio in ananalyzer region responsive to a magnetic field established across theregion, and an alternating electrical field established across theregion normal to the magnetic field, and collecting the spatiallyseparated ions of a given mass-to-charge ratio, the improvementcomprising introducing a sample to be analyzed into the analyzer regionand ionizing the sample by means of a hollow electron beam projectedacross the region parallel to the magnetic field.

The invention also includes in a mass spectrometer, the combinationwhich comprises an anaylzer chamber, means for establishing a magneticfield across the chamber in one direction, means for establising a highfrequency alternating electrical field across the chamber transverse tothe magnetic field, means for introducing a sample to be analyzed, meansfor developing a hollow electron beam and projecting it across thechamber parallel 'to'the magnetic field for ionizing the sample, :and acollector electrode disposed in the chamber remote "from the electronbeam for collecting ions.

To best take advantage of'the hollow ionizing electron beam, it isnecessary to provide a plurality of collector electrodes spaced aroundthe beams. With'this arrangement, ions formed anywhere around the beamwill have approximately the same radial distance to travel in reaching acollector electrode.

.Any number of meansmay be employed for developing a hollow electronbeam. Although alternative means are illustrated and hereinafterdescribed for accomplishing this, the invention is not primarilydirected to the means for developing such a beam but to the combinationof such means in 'a mass spectrometer of a given type.

The invention will be more clearly understood by reference to thefollowing detailed description thereof taken in conjunction with theaccompanying drawing wherein:

Fig. 1 is a diagram representing a crosssec- "tion of an area traversedbya magnetic and alternating electrical field normal to the mag- "neticfield, the section beingtaken through the .magnetic field and showingthe conventional type of electron beam paralleling the magnetic field;

.Fig. 2 is a similar transverse section through crossed magnetic.andelectrical fields showin ,the relationship of .the. electron.beam.in accordance with .the present inventionto. suchfields;

.Fig. .:3 .is a sectional .elevationof .oneform .of .apparatus inaccordance with the. invention;

. Fig. A .is-a. horizontalsection taken on .the .line

.4-4 of Fig. 3;

Fig. 5 isa perspective-view of one form ofelectron gun for developing ahollow electron beam;

Fig. 6 is a section taken on the line 66 of .Fig. 5; and

Fig. 7 is an:elevation-ofanother formof electron gun for developing ahollow electron beam in accordance with the invention.

.Fig. 1 is a diagram showing the relationship of a conventional electronbeam ID of radius To to an alternating electrical field represented byan arrow E normal to the axis of the beam and to a magnetic fieldparallel'to the axis of the beam, 1. e. perpendicular'to the plane ofthedrawing. 'A collector electrode [2 is disposed adjacent the periphery ofthe space traversed by the fields as is conventional'in the practice ofithis -form-of mass spectrometry. The electrode .small gaps separatingtheir adjoining edges.

I2 is spaced at a radius 73 from the center of beam ID.

The significance of Fig. 1 becomes more apparent upon examination ofFig. 2, which shows a hollow electron beam [4 in accordance with theinvention transverse to an alternating electrical field represented bythe arrow E and parallel to a magnetic field perpendicular to the planeof the drawing. In this instance, a plurality of collector electrodes I2are disposed around the beam adjacent the periphery of the crossed.fields at a radius 12 from the center of the beam.

If an electron beam in the form of a hollow cylinder of outer radius nand thickness t of .the type .shownjn Fig. 2 has collector electrodesdisposed at a radius m from the axis of the beam, the'maximum radial ionpath length is equal to the. function (rzr1)+t. The area of the electronbeam which determines the sensitivity of the instrument is given by theexpression or approximately '21rrit. By keeping t and the function(Ta-r1) small, ion path lengths may be kept small, not to exceed themaximum radial ion path lengths (T3) in the system of Fig. 1, whilesensitivity may be greatly increased by making the radius n of theelectron beam fairly large.

The limits on the radius ofthe'electron'beam are determined 'only by thelimits on the cost of developing a'magnetic field across the spaceparallel to the beam, since it follows that the radius r2 mustalso beincreased along with n in order to achieve a'glven path length necesaryfor the spatial separation of resonant from non-resonant ions. In short,there is a minimum value of the function (r2r1) necessary to effectiveseparation. I have found that by using a hollow electron beam of thetype shown in Fig. 2, a material increase in sensitivity may be achievedbefore the attendant increase in magnet cost becomes appreciable. InFigs. 1 and 2 the maximum ionic path lengths [(72-1'1) in Fig. 2,'and(T3) in Fig. 1] are the same. .However, the sensitivity of an instrumentas shown in Fig. 2 is more than six times as great as'the sensitivity ofan instrument as shown in Fig. 1 at the expense only of an increase inmagnetic gap area of about 50%. The advantages of the present inventionare apparent from the foregoing discussion.

One form of instrument .in accordance with the invention is showninsectional elevation'in Fig. 3. .Referring .to these figures, the.instru- .ment comprisesanenvelope Zllhaving magnetic ;pole pieces2I,.22 forming opposite end walls of the envelope. A pair ofsemi-cylindrical electrodes 24, 25 are mounted within the envelope andare oriented therein to define a substantially cylindrical analyzerregion 25. The electrodes 24, 25 are insulated from each other as by theA pair of electrodes 28, 29 are mounted adjacent and across oppositeends of the electrodes 24, 25 substantially enclosing the analyzerregion 26. The electrodes 28, 29 are insulated from the semi-cylindricalelectrodes 24, 25 as by the illustrated gaps. A number of collectorelectrodes 32 are mounted through the walls of electrodes 24,25 and areinsulated therefrom. The collector electrodes are connected througha'com- 'mon lead to an amplifying and recording network 34'across agrounded resistor 36. The electrodes need not be maintained at groundpotential, the invention having nothing to do with the niceties ofpotential relationships between the various elements of the instrument.

Envelope 20 is provided with an evacuating line 38 for connection to avacuum pump (not shown) and with a gas inlet line 40 for admitting asample of gas to be analyzed into the envelope. Gas entering theenvelope through the inlet line 4!) finds its way into the analyzerregion by diifusion through the gaps separating the four electrodeswhich define the region.

The end electrodes 28, 29 are provided with substantially centeredapertures 28A, 29A respectively, and an electron gun 42 is mountedadjacent the aperture ZBA exteriorly of the analyzer region fordirecting an electron beam 44 through the analyzer region. The beam 44passes out of the analyzer region through the aperture 29A in theelectrode 29 and is discharged at a target electrode 35 located adjacentthe aperture 25A exteriorly of the analyzer region 26. Electron gun 42is of some convenient type for developing a hollow and preferablycylindrical electron beam as shown in the drawing. Various means forproducing such an electron beam are illustrated and describedhereinafter.

A high frequency oscillator 48 is connected across the semi-cylindricalelectrodes 24, 25, and a bias battery 59, connected to the endelectrodes 28, 29 and to ground, completes the usual circuit elements.

The operation of the apparatus just described is as follows. The systemis evacuated through evacuating line 38 and a sample of gas to beanalyzed is admitted to the envelope through inlet line 4!). The gasdifiuses into the analyzer region 26 and. upon intersecting, electronbeam 44 will be ionized. Under the influence of the magnetic field andthe transverse alternating electrical field established betweenelectrodes 24, 25, the ions will be set in motion with those ions of amass-to-charge ratio in resonance with the frequency of the alternatingfield traveling in spiral paths of ever increasing radius until theystrike on and discharge at one of the collector electrodes. The iondischarge is amplified and observed and provides means for determiningthe partial pressure of molecules in the gas sample from which the ionsof given mass-to-charge ratio were derived. The non-resonant ions willlikewise be set in motion and also in spiral paths, but will spiraloutwardly to a maximum radius short of the collector electrode and willthereupon collapse back to the origin (the electron beam) and so forth.

Under the influence of the magnetic field, the ions will tend to drifttoward the boundaries of the analyzer region in the direction of themagnetic field. This linear ion drift does not affect the orbital pathsof the ions but will interfere with the analysis if the ions are allowedto strike on and discharge at the end walls '28, 29. The small positivebias impressed on the end electrodes 28, 29 by the bias battery 56prevents such occurrence by repelling ions as they approach theseelectrodes. The result is that the ions oscillate in the analyzer regionparallel to the electron beam while they are driven in theircharacteristic spiral paths.

The electron gun S2 will emit electrons, but of itself is not capable ofpropelling the electrons across the chamber. A propelling potentialfield is necessary for this purpose. This field may be developed bymeans of an auxiliary electrode interposed between the electron gun andthe apertured electrode. More conveniently the electron gun is operatedat a negative potential so that the positively biased aperturedelectrode will provide the necessary propelling field. This is thearrangement assumed to exist in the drawing (Fig. 3) in which there isno auxiliary propelling electrode.

The operation of the apparatus as just described is in a senseconventional since the hollow electron beam does not affect the spatialseparation of the resonant and non-resonant ions nor does it affect theconfiguration of the motion of these ions. However, as explained above,the hollow electron beam does materially increase the sensitivity of theinstrument by pro viding a greater cross sectional area of ionizationwithout increase in the maximum radial ion path length. Such increase isto be avoided because of the attendant increase in the probability ofinter-ionic collisions with a resultant introduction of error. It is forthis reason that the same benefits can not be accomplished by increasingthe radius of a conventional uniform electron beam.

One means for producing a hollow electron beam is shown in perspectivein Fig. 5 and in section in Fig. 6, the latter figure being a section onthe line 65 of Fig. 5. This means comprises a hot cathode type ofelectron emitter including a cylindrical cathode 52 having a filamentheater 53 disposed therein. To this extent the illustrated electron gunis similar to the electron emitters discussed in Reich, Theory andApplication of Electron Tubes, first edition, page 24. The tube 52 has aflat end face 52A on which is disposed an annular layer 54 of anelectron emitter such as barium oxide, strontium oxide or the like.Electron tubes having flat end cathodes are known in the art, but to thebest of my knowledge the cathodes of all of such tubes are completelycovered by a layer of an electron emitter. When the oathode is heated bythe filament 53, electrons will be emitted only from the annular coating54 and in the form of a hollow beam. The beam is maintained incollimation by the magnetic field.

Another means for forming a hollow or tubular electron beam isillustrated in elevation in Fig. 7, which shows a portion of the endelectrode 28 (see Fig. 3) with the centered aperture 28A therein. Acircular filament 56 is oriented coaxially with respect to the aperture28A and is of somewhat smaller diameter than the aperture 28A. Electronsemitted from the filament 56 will be directed through the analyzerregion of the mass spectrometer in the form of a tubular beam by reasonof the circular configuration of the filament, the orientation thereofwith respect to the circular aperture 28A, and the collimating action ofthe magnetic field.

Other means may be developed for producing hollow electron beams nowthat a purpose for such beams has been discovered. The invention is notintended to be limited to any specific means for accomplishing this. Itis in the broader aspects of the use of a hollow electron beam in thepractice of mass spectrometry and the combination of means for producingsuch a beam in a mass spectrometer of the specific type described thatthe invention is particularly directed.

I claim:

1. In a mass spectrometer, the combination which comprises an analyzerchamber, means for establishing a magnetic field across the:chamber,imeans for :establishing a high frequency alternating:electrical field across the chamber transverse to the magnetic field,means for introducing asample to be analyzed, means for developing ahollow electron beam and projecting it across the chamber parallel tothe magnetic field for ionizing the sample, anda collector electrode forcollecting ions disposed in the chamber remote from the electron beam.'2. In a mass spectrometer, the combination which comprises an analyzerchamber, means for establishing a magnetic field across the chamber,means for establishing a high frequency alternating electrical fieldacross the chamber transverse to the magnetic field, means forintroducing "a sample to be analyzed in a substantiallyun-ionized'state, means for developing a hollow electron beam'andprojecting it across the chamber parallel to the magnetic field and inthe plane of symmetry of the electrical field for ionizing the sample,and a collector electrode for collecting ions disposed in the chamberremote from the electron beam.

3. In a mass spectrometer, the combination which comprises an analyzerchamber, means for establishing a magnetic field across the chamber, afirst pair of electrodes disposed on opposite sides of the chamber,means including said pair of electrodes for establishing a highfrequency alternating electrical field across the chamber transverse tothe magnetic field, a second pair of electrodes mounted at opposite endsof said first pair of electrodes and across the ends or the chamber,each of said second pair of electrodes having a substantially centeredaperture, means for introducing a sample to be analyzed, an electron gunadapted to develop a hollow electron beam disposed exteriorly of thechamber adjacent the aperture in one of said second pair of electrodes,an electron target disposed exteriorly of'the chamber adjacent theaperture in the other one of said second pair of electrodes, whereby ahollow electron beam is projected from the gun to the target across thechamber and parallel to the magnetic field, and a collector electrodedisposed in the chamber remote from the electron beam for collectingions.

4. Apparatus according to claim 3 wherein the electron gun comprises alooped electron emitting :filament disposed coaxially with respect tosaid aperture and being of smaller diameter than said aperture.

5. Apparatus according to claim 3 wherein the electron gun comprises atubular cathode having a flat end face, a heater filament disposed inthe cathode for heating said fiat end face, and an annular ring of anelectron emitting substance coated 'on said end face.

6. In a mass spectrometer, the combination which comprises an analyzerchamber, means for establishing a magnetic field across the chamber inone direction, means for establishing a high frequency alternatingelectrical field across the chamber transverse to the magnetic field,meansfor introducing a sample to be analyzed, means for developing ahollow electron beam and projecting it across the chamber parallel tothe magnetic field for ionizing the sample, and a plurality of collectorelectrodes disposed in the chamber remote from the electron beam andoriented around the beam.

7. In a mass spectrometer including an ionization chamber, means forestablishing a magnetic field across the chamber in one direction, meansfor establishing an alternating electrical field across the chambernormal to the magnetic field, and a collector electrode, the improvementcomprising a gas inlet for admitting a substantially un-ionized gassample into the chamber for analysis, and means for developing a hollowelectron beam and projecting it across the chamber parallel to themagnetic field for ionizing the gas sample.

8. In a mass spectrometer, the combination which comprises an analyzerchamber, means for establishing a magnetic field across the chamber,means for establishing a high frequency alternating electrical fieldacross the chamber transverse to the magnetic field, means forintroducing a sample to be analyzed, means for developing a hollowelectron beam and projecting it across the chamber parallel to themagnetic field for ionizing the sample, and means for collecting ions inthe chamber remote from the electron beam.

CHARLES F. ROBINSON.

No references cited.

